This invention relates to an image signal processing method and an apparatus for practicing the method, namely, an image signal processing apparatus, and more particularly to an image signal processing method in which removal of noise components from image signals is carried out by averaging, and an apparatus for practicing the method.
An image processing technique of this type is important for image pickup devices in electronic cameras, such as in VTRs (video tape recorders) or for devices which supply reproduced image signals to a monitor.
A conventional image signal processing apparatus, as shown in FIG. 5, comprises: an A/D (analog-to-digital) converter to which picture element signals S are output, with predetermined timing, by the picture elements of an image pickup element provided, for instance, in an electronic camera; an adder 2 which adds picture element data D.sub.S output by the A/D converter 1 to picture element data D.sub.L provided by a first frame memory 3 or a second frame memory 4, and outputs operation data D.sub.N which is obtained by dividing the result of addition by two (2). the first and second frame memories 3 and 4 each having a data storage capacity of one frame; and switching circuits 5 and 6. The switching circuit 5 transmits the operation data D.sub.N to the first memory 3 or the second frame memory 4. The switching circuit 6 transmits the output data of the first frame memory 3 or that of the second frame memory 4, as the picture element data D.sub.L, to the adder 2.
The operation of the image signal processing apparatus thus organized will be described.
It is assumed that picture element data for the frame one vertical scanning period have previously been stored in the second frame memory 4. After the switching circuits 5 and 6 are connected to the first frame memory 3 and the second frame memory 4, respectively, as shown in FIG. 5, application of a video signal S to the A/D converter 1 is started. As a result, picture element data D.sub.S is output sequentially, for instance, by the point sequential scanning of the image pickup elements, and this data are added to picture element data D.sub.L which are read out of the second frame memory 4, beginning with the top address, in synchronization with the point sequential scanning. The results of addition are divided by two (2), and are sequentially stored in the first frame memory 3 beginning with the top address. That is, operation data D.sub.N for one frame according to the following equation (1) are stored in the first frame memory 3: EQU D.sub.N (i)=1/2(D.sub.S (i)+D.sub.L (i)) (1)
where i is the memory address.
Thereafter, the switching circuits 5 and 6 are operated to select the second frame memory 4 and the first frame memory 3, respectively. As a result, similarly to the above-described case, picture element data D.sub.S for the frame in the next vertical scanning period are added to picture element data D.sub.L read out of the first frame memory 3, and the results of addition are divided by two (2), the resultant operation data D.sub.N being stored in the second frame memory 4. Thus, in conjunction with equation (1), ##EQU1##
where n is the number of processing operations (in this case, n=2).
As was described above, one pair of frame memories 3 and 4 are provided so that new picture element data D.sub.S are added to old picture element data D.sub.L. As a result, asynchronous noise components included in these data are removed so that the quality of the reproduced image is improved.
However, the above-described signal processing technique suffers from a difficulty that, as it is required to use a pair of large capacity frame memories, the image signal processing apparatus is unavoidably bulky.